Gabriel,
You are handling this the best you can, which in my opinion is pretty darn good. Its a fine line between being a door mat and unconditional love, everyone has to strike their own balance. I believe that the WAS is riding an emotional rollercoaster too. You can never predict what willl come out of their mouth. The best thing you can do is not let it affect you, and of course you do this by not reacting negatively to their outbursts.
One of the most valuable lessons I've learned from DBing is to pick my battles. I used to do battle on every little thing. Now I let more issues go. I feel so much better. I agree with Jo, try not to focus on MIL. If they are spending their time talking negative about you, well they are just wasting their time. They obviously dont know the same guy, I read about on this board everyday.
Keep your chin up.
You'll figure the balancing act out, in time.
Hope.